Physicists have long grappled with the mystery of dark matter, an enigmatic substance that makes up approximately 27% of the universe’s mass-energy content. Unlike ordinary matter, which comprises the stars, planets, and life forms we observe, dark matter does not interact with light, making it invisible to conventional telescopes. Despite nearly a century of theoretical proposals and experimental searches, the precise nature of dark matter remains elusive.
On large cosmic scales, dark matter is thought to be “cold”—moving slowly relative to the speed of light—and non-collisional, meaning it does not interact with ordinary matter in ways familiar to us. These properties make cold dark matter an excellent candidate for explaining the large-scale structure of the universe. However, on smaller scales, dark matter may behave differently, leaving subtle but observable imprints on the early universe.
The challenge lies in distinguishing the effects of dark matter from those of baryonic matter, which includes protons, neutrons, and other components of ordinary matter. Both types of matter influence the formation of small-scale structures in the early universe, complicating efforts to isolate the signature of dark matter. Discrepancies between theoretical predictions and observations at galactic and sub-galactic scales persist, raising questions about whether cold dark matter can fully account for these phenomena or if new physics is required.
A team led by Jo Verwohlt at the University of Copenhagen has proposed an innovative method to detect dark matter signatures by examining deeply redshifted hydrogen emissions from the universe’s first stars and galaxies. Their findings, published in Physical Review D, focus on the 21-centimeter hydrogen spectral line, a key observational tool for studying the early universe. This line arises when a neutral hydrogen atom transitions between hyperfine energy states, emitting or absorbing a photon with a wavelength of 21 centimeters.
The team’s approach leverages theoretical models of “dark radiation,” a hypothetical form of radiation that could mediate interactions between dark matter particles. Analogous to how photons facilitate electromagnetic forces, dark radiation—if it exists—would enable dark matter particles to interact with one another. One candidate for dark radiation is the sterile neutrino, a particle that does not interact with the known forces of the Standard Model.
In the early universe, interactions between dark radiation and dark matter could have generated heat, raising the temperature of dark matter. This warming might have facilitated the formation of “dark matter halos,” regions where dark matter is gravitationally bound and decoupled from the universe’s overall expansion. These halos could have experienced cycles of collapse and resistance, producing density fluctuations known as “dark acoustic oscillations.” While these oscillations would have faded quickly, they could have influenced the environment around the “cosmic dawn,” when the first galaxies formed from primordial gas.
Verwohlt’s team modeled how these early oscillations might have affected the 21-centimeter hydrogen signal. During cosmic dawn, neutral hydrogen in intergalactic space would have absorbed or emitted 21-centimeter photons against the backdrop of the cosmic microwave background. The resulting signal would carry imprints of dark matter damping at small scales, revealing the presence and properties of dark acoustic oscillations.
The researchers employed an “effective theory of structure formation,” a versatile framework for predicting the development of cosmic structures under various dark matter scenarios. They also incorporated models of star formation to link the 21-centimeter signal to the rate of galaxy formation. By simulating different dark matter scenarios, the team demonstrated that variations in the 21-centimeter signal could reveal the existence of dark acoustic oscillations and help distinguish between competing models of dark matter.
Their study found that the Hydrogen Epoch of Reionization Array (HERA), a cutting-edge radio telescope in South Africa, could play a pivotal role in this research. According to their calculations, HERA would need approximately 18 months of continuous observations to detect the redshifted 21-centimeter signal from the cosmic dawn and determine whether dark acoustic oscillations exist. Such observations could provide unprecedented insights into the microphysical properties of dark matter and its role in cosmic evolution.
If successful, this method could revolutionize our understanding of dark matter, offering a new way to probe its interactions and properties. By illuminating the role of dark matter in the universe’s earliest epochs, these findings could bridge the gap between theoretical predictions and observational evidence, bringing physicists closer to unraveling one of the universe’s greatest mysteries.